Arsenic coal metamorphism

Coals are sedimentary and sometimes metamorphic rocks that contain at least 50 wt % and 70 vol % combustible organic materials, which are often valuable sources of energy (Craig, Vaughan and Skinner, 2001), 503. Unlike many other sedimentary and metamorphic rocks, coals often contain substantial pyrite and other sulfide minerals. The organic matter and sulfide minerals in coals frequently have strong affinities for arsenic. 

In the subsurface, arsenic may readily accumulate in coal. Specifically, groundwater percolating through volcanic deposits may leach arsenic and transfer it to underlying coal beds during or after coalification. Contact metamorphism and any associated hydrothermal fluids are other processes that sometimes locally enrich arsenic in coal seams. At a site in British Columbia, a Cretaceous bituminous coal seam was contact metamorphosed by a mafic dike. The contact temperatures were about 700-900 °C, which could volatilize arsenic and/or dissolve it into associated hydrothermal fluids. The arsenic was then transported and deposited into cooler sections of the seam (Yudovich and Ketris, 2005), 163. 

In the bituminous coals of the US Illinois and Appalachian basins, arsenic primarily occurs in pyrite. The arsenian pyrite probably originated from subsurface fluids that existed about 270 million years ago during the formation of the Ouachita and Appalachian mountains (Goldhaber, Lee and Hatch, 2003). The arsenic-bearing fluids in the midcontinent Illinois Basin were primarily brines derived from surrounding sedimentary basins that were also responsible for the formation of the Mississippi Valley lead-zinc deposits. In contrast, the fluids that were responsible for the arsenian pyrites in the Appalachians (especially in the coals of the Warrior Basin of Alabama) were metamorphic and not as saline as those in the midcontinent (Goldhaber, Lee and Hatch, 2003). 

The second part of the paper addresses arsenic enrichment in coal of the Appalachian Basin and adjacent Appalachian Mountains in the eastern United States (areas 3 and 4 in Fig. 1). In this regional setting, elevated crustal arsenic concentrations in rocks are vell documented, but processes associated vith the original introduction of arsenic into coal and metamorphic rocks have not been systematically studied. However, the Appalachian region is very important because here both natural weathering and mining can potentially lead to dispersion of arsenic into the local environment. 

Evidence Supporting A Genetic Link Between Metamorphic Fiuid And Arsenic Enrichment In Aiabama Coal... 

The distribution of elevated arsenic concentrations in Alabama coal basins bears on the transport path of the arsenic. If arsenic was carried in metamorphic fluids that were driven westward by tectonic forces, then the fold belt, which lies between the Warrior basin and the presumed source of the fluids in metamorphic rocks should have been along this flow path. Significantly, chemical analyses of coal from the Cahaba coalfield, which is located in the fold belt, confirm high arsenic concentrations in these coal beds (Fig. 15). 

In summary, the evidence presented above is best interpreted as indicating that the arsenic enrichments found in Alabama bituminous coal was introduced following deposition and some (perhaps considerable) burial by the distal portion of a regional westward-directed metamorphic fluid-flow system. 

ENVIRONMENTAL IMPACT OF ARSENIC IN COAL AND METAMORPHIC ROCKS. 

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